This invention relates to fluorinated copolymer membranes useful as a precursor membrane for making a fluorinated cation exchange membrane for use in electrolysis of an aqueous solution of a halogenated alkali metal. More particularly, this invention relates to fluorinated copolymer membranes and the production of the same, the membrane being comprised of a first layer containing pendant sulfonylchloride groups and a second layer containing sulfonylfluoride groups.
In the industry of producing caustic soda and chlorine by electrolyzing salt, the ion-exchange membrane process has attracted special interest recently because this process is more advantageous than the conventional mercury and diaphragm processes in view of its effectiveness for preventing pollution and saving energy. The quality of the product of this ion-exchange membrane process is as satisfactory as that of the mercury process. Many fluorinated cation exchange membranes, therefore, have been proposed for this purpose, including those containing substituents such as sulfonic acid groups, carboxylic acid groups, sulfonamide groups and the like.
Among these membranes, those such as disclosed in Japanese Published unexamined patent application Ser. Nos. 24176/1977, 24177/1977, 104583/1978, 116287/1978 and 6887/1979 are preferable in view of their performance and chemical and physical resistance. They are the membranes in which carboxylic acid groups have been introduced by chemical treatment to the one surface of a fluorinated cation exchange membrane containing sulfonic acid groups.
Considering, as a whole, such features of the resulting membranes as performance, ease in controlling density gradient of a carboxylic acid group, simplicity of reaction steps, reagent cost, ease in suppressing side-reactions, possibility of changing pendant groups to perfluoro-groups, physical damage which might be introduced during chemical treatment, etc., it is noted that the most preferred one is that disclosed in Japanese Published unexamined patent application Nos. 24176/1977 and 24177/1977 (which have been combined and correspond to U.S. Pat. No. 4,151,053), in which carboxylic acid groups: --OCF.sub.2 CO.sub.2 M (wherein M is H, a metal or ammonium group) have been incorporated in one surface layer of the resulting fluorinated cation exchange membrane containing sulfonic acid groups: --OCF.sub.2 CH.sub.2 SO.sub.3 M (wherein M is the same as in the above).
Those fluorinated cation exchange membranes mentioned above have been produced by treating with a reducing agent fluorinated copolymer membranes having pendant groups including the group: --OCF.sub.2 CF.sub.2 SO.sub.2 X [wherein X is halogen, OH, allyl, alkyl or OZ (Z being a metal, ammonium, allyl or alkyl)] or the group: --OCF.sub.2 CF.sub.2 SO.sub.2).sub.2 O, or preferably the group containing X being Cl or F, most preferably, the group of where X is Cl.
In order to form carboxylic acid groups on one surface of the membrane, the membrane has to be fixed with a frame and then only the one surface is assumed to contact with a reducing agent. In practicing this process on an industrial scale, it is necessary to mass-produce these membranes by manual work and this makes this process less economical.